Well, I'm using Phorum at the mo.
http://www.phorum.org/
The board does not have a IGNORE USER option, but I really do need it right now. I tried to write some filters but ended up hiding only the user's username and signature (duh!), not the messages. Does anyone know what to do?

No, this can't be done with CSS, consequently you can't do it with Stylish or Adblock Plus either. You'll need some program that can really cut out parts of the HTML code - like Proxomitron._________________Adblock Plus developer
http://adblockplus.org/